WO2014196207A1 - Wavelength conversion light source - Google Patents

Wavelength conversion light source Download PDF

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Publication number
WO2014196207A1
WO2014196207A1 PCT/JP2014/003019 JP2014003019W WO2014196207A1 WO 2014196207 A1 WO2014196207 A1 WO 2014196207A1 JP 2014003019 W JP2014003019 W JP 2014003019W WO 2014196207 A1 WO2014196207 A1 WO 2014196207A1
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wavelength
light
signal light
light source
semiconductor laser
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PCT/JP2014/003019
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French (fr)
Japanese (ja)
Inventor
忠永　修
西田　好毅
宮澤　弘
都巳 草薙
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Ｎｔｔエレクトロニクス株式会社
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Publication of WO2014196207A1 publication Critical patent/WO2014196207A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3534—Three-wave interaction, e.g. sum-difference frequency generation
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/35—Non-linear optics
    • G02F1/353—Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
    • G02F1/3544—Particular phase matching techniques
    • G02F1/3548—Quasi phase matching [QPM], e.g. using a periodic domain inverted structure

Definitions

  • the present invention relates to a wavelength conversion light source, and more specifically to a wavelength conversion light source that generates light in the mid-infrared wavelength region suitable for gas sensing and spectroscopy.
  • a waveguide type wavelength conversion element that periodically modulates the nonlinear optical constant and uses quasi-phase matching from a practical viewpoint.
  • a method of alternately inverting the sign of the nonlinear optical constant, or substantially alternately arranging a portion having a large nonlinear optical constant and a portion having a small nonlinear optical constant can be considered.
  • the sign of the nonlinear optical constant corresponds to the polarity of the spontaneous polarization, and therefore the sign of the nonlinear optical constant can be reversed by inverting the spontaneous polarization.
  • a method for generating the mid-infrared wavelength region a method using difference frequency generation by a waveguide type wavelength conversion element using two semiconductor lasers and pseudo phase matching is known (for example, see Non-Patent Document 1). .
  • FIG. 1 is a schematic diagram showing a configuration of a light source using conventional wavelength conversion.
  • This light source is composed of a LiNbO 3 substrate 11 which is a nonlinear optical medium in which an optical waveguide 12 is formed, a multiplexer 15, and two semiconductor lasers (not shown).
  • the signal light 13 from the semiconductor laser and the pumping light 14 from another semiconductor laser are multiplexed by a multiplexer 15 and incident on an optical waveguide 12 formed on a periodically poled LiNbO 3 substrate 11.
  • a converted light 16 that is a difference frequency light between the signal light 13 and the excitation light 14 is generated.
  • the wavelength of the signal light (first incident light) is ⁇ 1
  • the wavelength of the converted light (idler light) is ⁇ 2
  • the wavelength of the excitation light (second incident light) is ⁇ 3
  • converted light of ⁇ 2 3.35 ⁇ m can be generated.
  • converted light of ⁇ 2 2.39 ⁇ m can be generated.
  • L represents the length of the nonlinear optical medium in the traveling direction of light.
  • the optical wavelength ⁇ 3 depends on the chromatic dispersion of the refractive index of the nonlinear optical medium, and is uniquely determined when the modulation period ⁇ 0 is determined. If the signal light wavelength ⁇ 1 or the pumping light wavelength ⁇ 3 is changed from a so-called quasi phase matching wavelength that satisfies the quasi phase matching condition, the conversion efficiency decreases according to (Equation 2) and (Equation 3).
  • FIG. 2 shows a change in conversion efficiency with respect to the phase mismatch amount.
  • the horizontal axis represents ( ⁇ 2 ⁇ / ⁇ 0 ) L / ⁇
  • the vertical axis is normalized with the maximum value of conversion efficiency being 1.
  • the wavelength band corresponding to the phase mismatch amount at which the conversion efficiency in FIG. When a 50 mm LiNbO 3 waveguide is used, it is about 7 nm when converted to a converted light wavelength in the 3.35 ⁇ m band, which is narrow.
  • the group velocity is the same between the 1.55 ⁇ m band signal light and the 2.39 ⁇ m converted light, so that the signal light wavelength is swept over a wide signal light range and the converted light is generated over a wide wavelength range. be able to.
  • semiconductor lasers that can be tuned to various wavelengths have been developed for use in optical communication equipment.
  • An external light source can be configured.
  • the hydrocarbon-based gas exhibits strong light absorption.
  • the wavelength range of light that can be generated from one light source is narrow in the prior art, so a wide wavelength range There was a problem that gas absorption could not be measured.
  • an excitation light source or a signal light source capable of sweeping a wide wavelength range is required.
  • a function capable of continuous wavelength sweeping is necessary to have a function capable of continuous wavelength sweeping.
  • an external resonator type laser light source including a semiconductor gain medium, a phase adjusting mechanism, and a wavelength limiting mechanism such as a grating can be considered.
  • DFB-LD Distributed FeedBack-Laser Diode
  • DFB-LD Distributed FeedBack-Laser Diode
  • sweeping of the injection current amount, temperature sweeping of the LD chip, etc. can be considered, but the variable wavelength range is very narrow, about 2 nm in the 1 ⁇ m band.
  • FP-LD Fabry Perot-LD
  • FP-LD Fabry Perot-LD
  • a light source for sensing a plurality of gases is required to have a contradictory characteristic that a dense sweep must be performed over a wide wavelength range, and a small and inexpensive light source.
  • An object of the present invention is to provide a small and inexpensive wavelength conversion light source capable of continuously generating light in the mid-infrared wavelength region over a wide wavelength range in the 3 ⁇ m band.
  • an embodiment of the present invention is a first semiconductor laser that outputs signal light having a wavelength ⁇ 1 , wherein the wavelength of the signal light is continuously in a predetermined signal light wavelength range.
  • the second semiconductor laser having a wavelength jump at a constant interval within the pumping light wavelength range, a multiplexer for multiplexing the signal light and the pumping light, and multiplexed by the multiplexer
  • a non-linear optical medium having a nonlinear optical effect that makes light incident and outputs converted light having a wavelength ⁇ 2 that is a difference frequency between the signal light and the excitation light, and the wave number corresponding to the signal light wavelength range is: It is wider than the wave number corresponding to the wavelength jump width.
  • FIG. 1 is a schematic diagram showing a configuration of a light source using conventional wavelength conversion
  • FIG. 2 is a diagram showing a change in conversion efficiency with respect to the phase mismatch amount
  • FIG. 3 is a diagram showing the wavelength dependence of the group refractive index of LiNbO 3
  • FIG. 4 is a diagram showing an example of characteristics of a semiconductor laser that generates excitation light in the present embodiment.
  • FIG. 5 is a diagram illustrating an example of characteristics of a semiconductor laser that generates signal light in the present embodiment;
  • FIG. 6 is a diagram illustrating a conceptual diagram of the configuration according to the first embodiment of the invention.
  • FIG. 7 is a diagram showing the characteristics of wavelength conversion by the LiNbO 3 waveguide when the wavelengths of the signal light and the excitation light are changed in Example 1 of the present invention
  • FIG. 8 is a diagram illustrating a relationship between converted light and signal light in the first embodiment.
  • FIG. 9 is a diagram illustrating a conceptual diagram of a configuration according to Embodiment 2 of the present invention.
  • FIG. 10 is a diagram showing a conceptual diagram of a configuration according to Embodiment 3 of the present invention.
  • FIG. 11 is a diagram illustrating an example of characteristics of a light source that generates excitation light in Example 3.
  • FIG. 12A is a diagram showing the wavelength ranges of signal light and excitation light in Example 4
  • FIG. 12B is a diagram illustrating the wavelength ranges of signal light and converted light in the fourth embodiment.
  • the inventor of the present invention diligently studied the configuration of a light source capable of obtaining an output over a wide wavelength band in a wavelength range of 3 ⁇ m where various gases exhibit large absorption.
  • semiconductor lasers are used for the excitation light source and the signal light source.
  • a DFB-LD with easy wavelength sweep is used as the signal light source, it is impossible to sweep in a wide wavelength range, so a light source capable of discontinuously varying the wavelength is used as the excitation light source.
  • a 1.5 ⁇ m band DFB-LD is used as the first semiconductor laser capable of dense wavelength sweeping in a narrow wavelength band, and wavelength sweeping is possible over a wide wavelength band allowing mode hops rather than continuous.
  • an FP-LD in the 1.0 ⁇ m band (1.0-1.1 ⁇ m range, more specifically 1.02-1.08 ⁇ m range) is used.
  • Phase matching conditions over a wide wavelength range by generating the difference frequency by injecting the signal light from the first semiconductor laser and the excitation light from the second semiconductor laser into a LiNbO 3 optical waveguide having periodic polarization inversion It was discovered that light in the mid-infrared wavelength region can be generated over a wide wavelength range in the 3 ⁇ m band. The operation principle will be described below.
  • ng is a group refractive index given by the following equation.
  • FIG. 3 is a diagram showing the wavelength dependence of the group refractive index of LiNbO 3 .
  • the horizontal axis in FIG. 3 represents the wavelength, and the vertical axis represents the group refractive index.
  • the solid line in FIG. 3 represents the result when using bulk crystal LiNbO 3 and the dotted line represents the result when using LiNbO 3 having a waveguide structure.
  • the converted light and the group refractive index coincide with each other when the center wavelength is about 3.5 ⁇ m and the range is 3.39 to 3.6 ⁇ m. Therefore, group velocity matching can be used by setting the signal light wavelength to about 1.50 ⁇ m. This makes it possible to convert the excitation light in the wavelength region of 1.02-1.08 ⁇ m using the FP-LD into converted light over a wide band centered on 3.5 ⁇ m.
  • the waveguide When using a waveguide to obtain 3 ⁇ m converted light by generating a difference frequency between 1.0 ⁇ m band excitation light and 1.55 ⁇ m band signal light, the waveguide has the longest wavelength among the three interacting wavelength bands.
  • the core size of the waveguide is set so that the single mode condition is almost satisfied at 3 ⁇ m.
  • the fundamental mode used for wavelength conversion is strongly confined in the optical waveguide, and the influence of structural dispersion caused by the shape of the waveguide is almost eliminated. I do not receive it.
  • the group refractive index of LiNbO 3 having a waveguide structure hardly changes in the 1.0 ⁇ m band and the 1.55 ⁇ m band compared to the bulk case.
  • the equivalent refractive index of the waveguide mode is greatly affected by the structural dispersion due to the waveguide structure. Therefore, the characteristics due to the coincidence of group velocities can be analyzed by calculating the group refractive index in the 3 ⁇ m band.
  • the wavelength dependence (structural dispersion) of the refractive index is increased by providing the waveguide structure.
  • the group refractive index in the 3 ⁇ m band is larger than that of the bulk crystal. Therefore, as can be seen from FIG. 3, the wavelength band of the converted light that provides the coincidence of group velocities with respect to the wavelength band of 1.02-1.08 ⁇ m, which is a typical wavelength variable range of a light source using a Yb fiber amplifier, is a short wavelength. Will shift to the side.
  • the converted light and the group refractive index coincide with each other in the range of 3.24 to 3.44 ⁇ m (the center wavelength is about 3.34 ⁇ m) with respect to the excitation wavelength.
  • the sweep width of the difference frequency light (converted light) when the group velocity matching is specifically used will be described.
  • the allowable wavelength range of the signal light in the 1.5 ⁇ m band is narrow, and even if the wavelength of the signal light is swept, the wavelength range of the difference frequency light in the 3 ⁇ m band is about 7 nm. It is.
  • the wavelength range of the difference frequency light in the 3 ⁇ m band is about 123 nm.
  • the range of the wavelength sweep of the excitation light using the group velocity matching is approximately 17 times wider than the wavelength sweep of the signal light.
  • the wavelength sweep width is further increased according to Equations 2 and 3, and when the excitation light wavelength is swept, the wavelength range of the difference frequency light in the 3 ⁇ m band is about 197 nm.
  • the allowable wavelength range is widened by shortening the waveguide length, but at the same time, the conversion efficiency is lowered.
  • the decrease in conversion efficiency can be compensated by combining a pumping light source with a Yb fiber amplifier.
  • variable wavelength range of the converted light is targeted to be 100 cm -1 or more (wavelength range of 122 nm or more), either the signal light or the excitation light must be varied by 100 cm -1 or more.
  • the variable wavelength range of converted light with a center wavelength of 3.34 ⁇ m is converted to wave number and the target is 100 cm ⁇ 1 or more (wavelength range of 110 nm or more)
  • the signal light of 1.55 ⁇ m band is 24 nm or more, or 1.06 ⁇ m band It must be tunable in the wavelength range of 11 nm or more with excitation light.
  • FIG. 4 shows an example of characteristics of a semiconductor laser (FP-LD) that generates excitation light.
  • FP-LD semiconductor laser
  • about 20 nm wavelength jumps are observed at regular intervals.
  • FIG. 5 shows an example of characteristics of a semiconductor laser (DFB-LD) that generates signal light.
  • DFB-LD semiconductor laser
  • This wave quantity has a wider range than the mode hop amount of the pumping light, and the mode hop of the pumping light can be compensated by the wavelength change of the signal light.
  • the phase matching range assuming that the element length of LiNbO 3 having a waveguide structure is 50 mm, the full width at half maximum of the phase matching range is 1.4 nm in the 1.55 ⁇ m band, which is sufficiently acceptable.
  • a wide excitation light variable range can be obtained by using a wavelength conversion element that matches the group velocity between the 1.05 ⁇ m band excitation light and the 3.4 ⁇ m band conversion light. Further, by using a pump light source having a mode hop such as FP-LD and a 1.55 ⁇ m band signal light source capable of continuous wavelength sweep, it is possible to continuously sweep over a wide wavelength range.
  • FIG. 6 shows a conceptual diagram of a configuration according to an embodiment of the present invention.
  • excitation light was generated by the FP-LD 102 having a variable wavelength around 1.065 ⁇ m.
  • the FP-LD 102 varies the wavelength of output light by a temperature control circuit 107 such as a Peltier element.
  • the DFB-LD 102 centered at 1.57 ⁇ m is used as the signal light source, and the wavelength of the output light is swept by controlling the drive current.
  • the pumping light and the signal light are combined by the fiber coupler 103 and incident on a LiNbO 3 crystal, which is a nonlinear optical medium having a waveguide 105 with a periodically poled structure, and is 3.3 ⁇ m mid-infrared light due to difference frequency generation. Converted light 106 was generated.
  • the wavelength conversion element 104 includes a nonlinear optical medium having a waveguide 105, a lens for inputting and outputting light, and the like.
  • the nonlinear optical medium is manufactured by a wafer bonding method shown in Non-Patent Document 3.
  • a ridge-type optical waveguide is formed by dicing using LiNbO 3 doped with 7 mol% of Zn for the core and LiTaO 3 for the clad.
  • the phase matching characteristic that is, the wavelength band characteristic capable of wavelength conversion is determined by the dispersion of the polarization inversion structure and waveguide structure of LiNbO 3 .
  • the size of LiNbO 3 forming the core was set to a thickness of 10 ⁇ m and a width of 14 ⁇ m.
  • the dispersion of the group refractive index of the waveguide in this waveguide size is as shown in FIG.
  • group velocity matching is satisfied between pump light having a center wavelength of 1.065 ⁇ m and converted light having a center wavelength of 3.31 ⁇ m, and phase matching is performed between the pump light, signal light, and converted light.
  • the device is designed so that is simultaneously satisfied.
  • FIG. 7 shows the characteristics of wavelength conversion by the LiNbO 3 waveguide when the wavelengths of the signal light and the excitation light are changed in this example.
  • the vertical axis indicates the normalized conversion efficiency
  • the horizontal axis indicates the converted light wavelength.
  • This is a conversion characteristic (solid line in FIG. 7) when the wavelength of the signal light is fixed to 1.570 ⁇ m and the wavelength of the FP-LD is varied in the range of 1.049 ⁇ m to 1.078 ⁇ m.
  • group velocity matching between the pumping light and the converted light can be used, as shown in FIG. 7, it is possible to efficiently generate the difference frequency over the entire wavelength variable range of the pumping light. Met. In this example, it was possible to output the converted light over a range of 120 nm from 3.24 ⁇ m to 3.36 ⁇ m.
  • the wavelength conversion characteristics when the wavelength of the excitation light is fixed to various wavelengths between 1.059 ⁇ m and 1.071 ⁇ m and the signal light wavelength is changed are also shown ((a) to (in FIG. 7). s)).
  • the wavelength-convertible band is 1.4 nm for the signal light wavelength and about 6 nm for the converted light wavelength.
  • the wavelength of the FP-LD shown in FIG. It was about 1/20 of the variable bandwidth.
  • This wavelength conversion characteristic curve has the same shape as the phase matching curve shown in FIG. 2 because the amount of phase mismatch in (Equation 2) changes almost linearly due to changes in signal light.
  • FIG. 8 shows the relationship between the converted light and the signal light in the first embodiment. Modulates the DFB-LD in the 1.55 ⁇ m band to fill the valley of the mode hop in the FP-LD in the 1.05 ⁇ m band.
  • the wavelength of the signal light from the DFB-LD has a sufficient wavelength change that can compensate for the mode hop of the pumping light only by sweeping in a narrow range from 1.5694 ⁇ m to 1.5708 ⁇ m. The wavelength can be swept continuously over a wide wavelength range.
  • the effectiveness of this example could be confirmed.
  • the absorption spectrum of the gas can be measured over a range of 120 nm.
  • the conversion efficiency of the wavelength conversion element used in this example is 20% / W. As a result of inputting 20 mW as signal light and 400 mW as excitation light into the device, an output of 0.8 mW was obtained, which was sufficient for gas detection applications.
  • LiNbO 3 doped with Zn was used as the core of the waveguide.
  • LiNbO 3 to which Zn is added it is possible to prevent optical damage especially when the intensity of the short-wavelength excitation light is large.
  • LiNbO 3 to which Mg, Sc, In or the like is added in addition to Zn for the purpose of preventing photodamage can also be used.
  • Zn used in this example is known to increase the refractive index of LiNbO 3 , but Mg and the like are known to decrease the refractive index of LiNbO 3 .
  • the wavelength at which group velocity matching is obtained changes because the structural dispersion changes due to the change in the wavelength dispersion of the material itself and the confinement of the waveguide.
  • the material configuration of the waveguide and the size of the core may be changed so as to obtain group velocity matching at a desired wavelength by actively utilizing this property.
  • the semiconductor laser is used as the light source for the pumping light and the signal light.
  • the pumping light can be amplified using a Yb fiber amplifier, or the signal light can be amplified using an Er-doped fiber amplifier. Both excitation light and signal light may be amplified using a fiber amplifier.
  • FIG. 9 shows a conceptual diagram of a configuration according to another embodiment of the present invention.
  • the configuration of the present embodiment is almost the same as the configuration of the first embodiment, but differs in that the pumping light is generated by amplifying the output of the tunable FP-LD 201 centering on 1.07 ⁇ m by the Yb fiber amplifier 208. To do.
  • the output of the DFB-LD 202 having a wavelength of 1.589 ⁇ m was amplified by an L-band Er-doped fiber amplifier 209 to generate signal light.
  • the excitation light and the signal light are combined by the fiber coupler 203 and incident on a LiNbO 3 crystal which is a nonlinear optical medium having a waveguide 205 with a periodically poled structure, and a mid-infrared centered at 3.275 ⁇ m by difference frequency generation.
  • Converted light 206 which is light, was generated.
  • the tunable FP-LD used in this example is tunable in the range of 1.064 ⁇ m to 1.076 ⁇ m.
  • group velocity matching between the pumping light and the converted light can be used, it is possible to efficiently generate the difference frequency over the entire wavelength variable range of the pumping light.
  • an arbitrary wavelength band is selected from the oscillatable wavelengths of the FP-LD, and broadband mid-infrared light is generated using group velocity matching with the converted light. Can do.
  • the signal light wavelength can be converted efficiently because the group velocity matching is achieved using a wavelength that can be used by the Er-doped fiber amplifier.
  • FIG. 10 shows a conceptual diagram of a configuration according to another embodiment of the present invention.
  • the FP-LD is used as a pumping light source.
  • the present embodiment is different in that an external resonator type semiconductor laser using a MEMS mirror is used as the pumping light source 301.
  • the excitation light source 301 includes a semiconductor amplifying medium 311 with one end face constituting a resonator of a normal semiconductor laser provided with a nonreflective coating, and a lens 312 and a MEMS mirror 313 on the end face side provided with a nonreflective coating. And an external resonator in which a grating 314 is disposed.
  • Such an external resonator type semiconductor laser has a simple configuration without a phase adjustment mechanism, and can realize a small and inexpensive light source by using a MEMS mirror as a drive unit for wavelength selection. it can.
  • the configuration of the light source of the third embodiment is a so-called Littrow arrangement, but the same function can be achieved in principle even with the Littman arrangement.
  • FIG. 11 shows an example of characteristics of a light source that generates excitation light in the third embodiment.
  • the center wavelength of the semiconductor laser is 1.064 ⁇ m, and the MEMS mirror is controlled to change the angle of light incident on the grating.
  • the reflected wavelength characteristic of the grating changes, and the output excitation light wavelength can be varied in the range of 1.059 ⁇ m to 1.069 ⁇ m. Since the external resonator is configured, the resonator length is longer than that of a normal semiconductor laser, and the interval at which the wavelength jump occurs is narrower than that of the FP-LD.
  • the output of the DFB-LD 302 having a wavelength of 1.570 ⁇ m was amplified by an L-band Er-doped fiber amplifier 309 to generate signal light.
  • the excitation light and the signal light are combined by the fiber coupler 303 and incident on a wavelength conversion element 304 having a LiNbO 3 crystal, which is a nonlinear optical medium having a waveguide 305 having a periodically poled structure, and centering around 3.30 ⁇ m.
  • Converted light 306 that is infrared light was generated.
  • the MEMS parameter for adjusting the angle of the MEMS mirror is controlled to include the mode hop, but the wavelength of the excitation light is changed in a wide wavelength range, and the driving current of the DFB-LD is continuously changed.
  • converted light in the 3 ⁇ m band can be obtained. Since the mode hop amount of the external cavity laser using the MEMS mirror is narrower than the mode hop amount of the FP-LD, the amount of change in the drive current of the DFB-LD is made smaller than that in the first embodiment. be able to.
  • FIG. 12A shows the wavelength ranges of the signal light and the excitation light in Example 4.
  • LiNbO 3 waveguide having a polarization inversion structure with a basic period ⁇ , a waveguide structure with a thickness of 10 ⁇ m and a width of 14 ⁇ m, and for a waveguide length L 50 mm, the basic period is changed from 27.177 ⁇ m to 28.607 ⁇ m, This is a conversion characteristic when the light wavelength is changed from 1.47 ⁇ m to 1.59 ⁇ m and the excitation light wavelength is changed.
  • FIG. 12B shows the wavelength ranges of signal light and converted light in Example 4.
  • group velocity matching can be realized at the excitation light wavelength of 1.028 ⁇ m and the converted light wavelength of 3.42 ⁇ m.
  • the signal light wavelength is determined by Equation 1 and is 1.47 ⁇ m.
  • the converted light changes over a range of 130 nm from 3.36 ⁇ m to 3.49 ⁇ m.
  • the excitation light wavelength is changed. Even if it is changed between 1.02 ⁇ m and 1.08 ⁇ m, a wide wavelength conversion band can be obtained.
  • the signal light wavelength is appropriately selected within the range of 1.47 ⁇ m ⁇ ⁇ 1 ⁇ 1.59 ⁇ m in the pumping light wavelength range of 1.02 ⁇ m ⁇ ⁇ 3 ⁇ 1.08 ⁇ m that can use the Yb fiber amplifier, the converted light (difference frequency) Light) wavelength 3.2 ⁇ m ⁇ ⁇ 2 ⁇ 3.5 ⁇ m can be obtained.
  • a semiconductor laser that generates a mode hop can be used as a light source that can sweep the excitation light wavelength widely.
  • the mode hop of the excitation light can be compensated by continuously sweeping the wavelength of the signal light in a narrow range, and as a whole, the 3 ⁇ m band has a wide wavelength band that allows continuous sweeping
  • the wavelength conversion light source can be realized.
  • the signal light can use a Tm-doped fiber amplifier in the 1.4 ⁇ m band, and can use an Er-doped fiber amplifier in the 1.5 ⁇ m band, thereby enhancing the output of the converted light.
  • the wavelength range of 1.53 ⁇ m to 1.59 ⁇ m is used for optical communication, and an inexpensive Er-doped fiber amplifier can be used.
  • the excitation light wavelength is in the wavelength range of 1.05 ⁇ m to 1.08 ⁇ m.
  • a semiconductor laser is used as a laser whose wavelength can be varied. However, since this wavelength band is available as a seed light source for a processing fiber laser, a light source can be easily configured. it can.
  • group velocity matching is performed between 1.0 ⁇ m band excitation light and 3 ⁇ m band converted light using the dispersion characteristics of the LiNbO 3 waveguide. Can be obtained. Thereby, a wavelength conversion element capable of generating converted light having a wide band in the 3 ⁇ m band can be realized.
  • the first semiconductor laser capable of continuously varying the center wavelength of the signal light in a predetermined signal light wavelength range, and the wavelength of the pump light can be varied in the predetermined pump light wavelength range.
  • a second semiconductor laser having wavelength jumps at regular intervals within the optical wavelength range, and a signal light wavelength range wider than the width of the wavelength jump is set, so that the wavelength of the excitation light and the wavelength of the signal light are changed.

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  • Nonlinear Science (AREA)
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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

Abstract

Provided are a wavelength conversion element and a wavelength conversion light source with which light in the mid-infrared wavelength region can be generated across a wide wavelength range in the 3-μm band. The wavelength conversion light source is equipped with: a first semiconductor laser, which outputs signal light of wavelength λ1, wherein the wavelength of the signal light is capable of varying continuously over a prescribed signal light wavelength range; a second semiconductor laser, which outputs excitation light of wavelength λ3, wherein the wavelength of the excitation light is capable of varying discontinuously with a prescribed wavelength variation width within a prescribed excitation light wavelength range; a multiplexer that multiplexes the signal light and the excitation light; and a nonlinear optical medium having a nonlinear optical effect, whereby the light multiplexed by the multiplexer is input, and converted light of wavelength λ2, which is the difference frequency of the signal light and the excitation light, is output.

Description

波長変換光源Wavelength conversion light source
　本発明は波長変換光源に関し、より具体的には、ガスのセンシングや分光に好適な中赤外波長域の光を発生する波長変換光源に関する。 The present invention relates to a wavelength conversion light source, and more specifically to a wavelength conversion light source that generates light in the mid-infrared wavelength region suitable for gas sensing and spectroscopy.
　従来、可視から中赤外あるいはTHz領域まで様々な波長領域の出力を有する半導体レーザが研究開発されている。しかしながら、例えば500-600nmの可視域、あるいは2-5μmの近赤外から中赤外の波長領域の出力を有する光源として、室温で簡易に使用できる光源は、現在のところ実現されていない。そこで、このような光源から直接発生させることが困難な波長領域の光を、非線形光学効果を用いた波長変換を利用して発生させる技術が知られている。 Conventionally, semiconductor lasers having outputs in various wavelength ranges from visible to mid-infrared or THz range have been researched and developed. However, a light source that can be easily used at room temperature as a light source having an output in the visible region of 500-600 nm or the near-infrared to mid-infrared wavelength region of 2-5 μm has not been realized at present. Therefore, a technique for generating light in a wavelength region that is difficult to directly generate from such a light source by using wavelength conversion using a nonlinear optical effect is known.
　波長変換素子としては様々な形態のものが利用可能であるが、実用的な観点から非線形光学定数を周期的に変調し、擬似位相整合を用いた導波路型の波長変換素子が最も有望である。非線形光学定数の周期変調構造を形成するためには、非線形光学定数の符号を交互に反転するか、あるいは非線形光学定数が大きい部分と小さい部分をほぼ交互に配置する方法が考えられる。LiNbO3のような強誘電体結晶においては、非線形光学定数の正負は自発分極の極性に対応するので、自発分極を反転することにより非線形光学定数の符号を反転することができる。中赤外波長域を発生させるための方法として、２つの半導体レーザおよび擬似位相整合を利用した導波路型波長変換素子による差周波発生による方法が知られている（例えば、非特許文献１参照）。 Various types of wavelength conversion elements can be used, but the most promising is a waveguide type wavelength conversion element that periodically modulates the nonlinear optical constant and uses quasi-phase matching from a practical viewpoint. . In order to form a periodic modulation structure having a nonlinear optical constant, a method of alternately inverting the sign of the nonlinear optical constant, or substantially alternately arranging a portion having a large nonlinear optical constant and a portion having a small nonlinear optical constant can be considered. In a ferroelectric crystal such as LiNbO 3 , the sign of the nonlinear optical constant corresponds to the polarity of the spontaneous polarization, and therefore the sign of the nonlinear optical constant can be reversed by inverting the spontaneous polarization. As a method for generating the mid-infrared wavelength region, a method using difference frequency generation by a waveguide type wavelength conversion element using two semiconductor lasers and pseudo phase matching is known (for example, see Non-Patent Document 1). .
　図１は、従来の波長変換を用いた光源の構成を示す概略図である。この光源は、光導波路１２が形成された非線形光学媒質であるLiNbO3基板１１、合波器１５、および２個の半導体レーザ（図示せず）から構成される。半導体レーザからの信号光１３および別の半導体レーザからの励起光１４を合波器１５で合波し、周期的に分極反転されたLiNbO3基板１１に形成された光導波路１２に入射して、信号光１３と励起光１４との差周波光である変換光１６を発生させる。信号光（第一の入射光）の波長をλ1、変換光（アイドラー光）の波長をλ2、励起光（第二の入射光）の波長をλ3とすると、これら３つの波長は次式を満たす。 FIG. 1 is a schematic diagram showing a configuration of a light source using conventional wavelength conversion. This light source is composed of a LiNbO 3 substrate 11 which is a nonlinear optical medium in which an optical waveguide 12 is formed, a multiplexer 15, and two semiconductor lasers (not shown). The signal light 13 from the semiconductor laser and the pumping light 14 from another semiconductor laser are multiplexed by a multiplexer 15 and incident on an optical waveguide 12 formed on a periodically poled LiNbO 3 substrate 11. A converted light 16 that is a difference frequency light between the signal light 13 and the excitation light 14 is generated. Assuming that the wavelength of the signal light (first incident light) is λ 1 , the wavelength of the converted light (idler light) is λ 2 , and the wavelength of the excitation light (second incident light) is λ 3 , these three wavelengths are Satisfy the formula.
Figure JPOXMLDOC01-appb-M000001
Figure JPOXMLDOC01-appb-M000001
　例えば、信号光波長λ1を1.55μm、励起光波長λ3を1.06μmとすれば、λ2=3.35μmの変換光を発生させることができる。また、信号光波長λ1を1.55μm、励起光波長λ3を0.94μmとすれば、λ2=2.39μmの変換光を発生させることができる。 For example, if the signal light wavelength λ 1 is 1.55 μm and the excitation light wavelength λ 3 is 1.06 μm, converted light of λ 2 = 3.35 μm can be generated. Further, if the signal light wavelength λ 1 is 1.55 μm and the pumping light wavelength λ 3 is 0.94 μm, converted light of λ 2 = 2.39 μm can be generated.
　信号光波長λ1における屈折率をn1、変換光波長λ2における屈折率をn2、励起光波長λ3における屈折率をn3、非線形光学定数の変調周期をΛ0とすると、（式２）で与えられる位相不整合量Δβ
Figure JPOXMLDOC01-appb-M000002
 に対して変換効率ηが（式３）で表される。 If the refractive index at the signal light wavelength λ 1 is n 1 , the refractive index at the converted light wavelength λ 2 is n 2 , the refractive index at the pumping light wavelength λ 3 is n 3 , and the modulation period of the nonlinear optical constant is Λ 0 , Phase mismatch amount Δβ given in 2)
Figure JPOXMLDOC01-appb-M000002
 The conversion efficiency η is expressed by (Equation 3).
Figure JPOXMLDOC01-appb-M000003
Figure JPOXMLDOC01-appb-M000003
　ここで、Ｌは非線形光学媒質の光の進行方向の長さを表す。 Here, L represents the length of the nonlinear optical medium in the traveling direction of light.
　（式３）より、変換効率ηは、Δβ=２π/Λ0のとき、最大となる。例えば、励起光波長λ3を固定して考えると、（式２）で与えられる位相不整合量Δβが、Δβ=２π/Λ0となるいわゆる擬似位相整合条件を満たす信号光波長λ1および励起光波長λ3は、非線形光学媒質の屈折率の波長分散に依存し、変調周期Λ0を決定すると実質的に一意に決定される。信号光波長λ1または励起光波長λ3を、擬似位相整合条件を満たすいわゆる擬似位相整合波長から変化させると、（式２）および（式３）に従って変換効率が減少してしまう。 From (Equation 3), the conversion efficiency η is maximized when Δβ = 2π / Λ 0 . For example, when the excitation light wavelength λ 3 is fixed, the signal light wavelength λ 1 and the excitation light that satisfy the so-called pseudo-phase matching condition where the phase mismatch amount Δβ given by (Equation 2) is Δβ = 2π / Λ 0. The optical wavelength λ 3 depends on the chromatic dispersion of the refractive index of the nonlinear optical medium, and is uniquely determined when the modulation period Λ 0 is determined. If the signal light wavelength λ 1 or the pumping light wavelength λ 3 is changed from a so-called quasi phase matching wavelength that satisfies the quasi phase matching condition, the conversion efficiency decreases according to (Equation 2) and (Equation 3).
　図２に、位相不整合量に対する変換効率の変化を示す。図２では、横軸は(Δβ-2π/Λ0)L/πを表し、縦軸は変換効率の最大値を１として規格化してある。例えば、励起光波長λ3を固定して信号光波長λ1を変換させたとき、図２の変換効率が最大値の半分となる位相不整合量に相当する波長帯域は、導波路長Ｌが50mmのLiNbO3導波路を用いた場合、3.35μm帯の変換光波長に換算すると約7nm程度であり狭い。 FIG. 2 shows a change in conversion efficiency with respect to the phase mismatch amount. In FIG. 2, the horizontal axis represents (Δβ−2π / Λ 0 ) L / π, and the vertical axis is normalized with the maximum value of conversion efficiency being 1. For example, when the pumping light wavelength λ 3 is fixed and the signal light wavelength λ 1 is converted, the wavelength band corresponding to the phase mismatch amount at which the conversion efficiency in FIG. When a 50 mm LiNbO 3 waveguide is used, it is about 7 nm when converted to a converted light wavelength in the 3.35 μm band, which is narrow.
　広い波長範囲において位相整合がとれる特殊な例が、非特許文献２に開示されている。具体的には、周期分極反転したLiNbO3を非線形材料として用い、λ1=1.55μmの信号光とλ3=0.94μmの励起光を入射して、λ2=2.39μmの変換光を発生させる。この例の場合は、1.55μm帯の信号光と2.39μmの変換光との間で群速度が一致するので、広い信号光範囲で信号光波長を掃引し、広い波長範囲で変換光を発生させることができる。 Non-patent document 2 discloses a special example in which phase matching can be achieved in a wide wavelength range. Specifically, using periodically poled LiNbO 3 as a nonlinear material, λ 1 = 1.55 μm signal light and λ 3 = 0.94 μm excitation light are incident to generate converted light of λ 2 = 2.39 μm . In this example, the group velocity is the same between the 1.55 μm band signal light and the 2.39 μm converted light, so that the signal light wavelength is swept over a wide signal light range and the converted light is generated over a wide wavelength range. be able to.
　特に、1.55μm帯では光通信用の機器に用いるために各種の波長可変が可能な半導体レーザが開発されており、これらを用いれば、2.39μm付近にて広い範囲で波長可変が可能な中赤外光源を構成することができる。 In particular, in the 1.55 μm band, semiconductor lasers that can be tuned to various wavelengths have been developed for use in optical communication equipment. An external light source can be configured.
　3μmの中赤外波長領域において、炭化水素系のガスは、強い光吸収を示す。各種のガスを高感度に検出するためには、これらの波長領域における光吸収を検出することが重要である。上述したλ1=1.55μmの信号光とλ3=1.06μmの励起光との組み合わせで3.35μm帯の中赤外変換光を発生させる場合、信号光と変換光の間で群速度は一致しない。このため、信号光波長を少し変化させただけで、擬似位相整合条件を満たさなくなり、上述のように3.35μm帯の変換光波長の帯域が約7nm程度と狭くなってしまうという問題があった。例えば、複数ガスをセンシングする必要がある場合や、ガスの複数の吸収ピークからなるスペクトルの計測を行う場合、従来技術では１つの光源から発生可能な光の波長範囲が狭いために、広い波長範囲に渡ってガスの吸収を計測することが出来ないという問題があった。 In the 3 μm mid-infrared wavelength region, the hydrocarbon-based gas exhibits strong light absorption. In order to detect various gases with high sensitivity, it is important to detect light absorption in these wavelength regions. When the mid-infrared converted light of the 3.35 μm band is generated by the combination of the signal light of λ 1 = 1.55 μm and the excitation light of λ 3 = 1.06 μm, the group velocities do not match between the signal light and the converted light. . For this reason, there is a problem that even if the signal light wavelength is slightly changed, the quasi-phase matching condition is not satisfied, and the converted light wavelength band in the 3.35 μm band becomes as narrow as about 7 nm as described above. For example, when it is necessary to sense a plurality of gases or when a spectrum consisting of a plurality of absorption peaks of gas is measured, the wavelength range of light that can be generated from one light source is narrow in the prior art, so a wide wavelength range There was a problem that gas absorption could not be measured.
　一方、複数のガスや複数の吸収線を検知できる広い波長範囲を掃引可能な波長変換光源を考えると、広い波長範囲を掃引可能な励起光光源もしくは信号光光源が必要となる。加えて、個々の吸収線の形状を観測するために、連続波長掃引できる機能を有する必要がある。広い波長範囲を連続掃引できる光源の一例としては、半導体利得媒質、位相調整機構、およびグレーティングなどの波長限定機構から構成される外部共振器型レーザ光源が考えられる。しかしながら、連続的に精度よく波長掃引を行うためには、波長限定機構の厳密な制御と位相調整機構の厳密な制御とが求められ、かつ環境温度の変化に対して高い安定性を保持するための機構も必要となる。従って、光源は、複雑、大型であり、非常に高価なものとなる。 On the other hand, when considering a wavelength conversion light source capable of sweeping a wide wavelength range capable of detecting a plurality of gases and a plurality of absorption lines, an excitation light source or a signal light source capable of sweeping a wide wavelength range is required. In addition, in order to observe the shape of each absorption line, it is necessary to have a function capable of continuous wavelength sweeping. As an example of a light source capable of continuously sweeping a wide wavelength range, an external resonator type laser light source including a semiconductor gain medium, a phase adjusting mechanism, and a wavelength limiting mechanism such as a grating can be considered. However, in order to perform wavelength sweep continuously and accurately, strict control of the wavelength limiting mechanism and strict control of the phase adjustment mechanism are required, and in order to maintain high stability against changes in environmental temperature. This mechanism is also required. Therefore, the light source is complicated, large, and very expensive.
　一方、連続波長掃引できる半導体レーザとして、一般的には、DFB-LD（Distributed FeedBack-LaserDiode）が知られている。DFB-LDの出力波長を可変するためには、注入電流量の掃引、LDチップの温度掃引などが考えられるが、可変波長範囲は１μm帯で2nm程度と非常に狭い。また、波長掃引範囲の広いレーザとして、FP-LD（Fabry Perot-LD）が知られている。しかしながら、FP-LDでは、波長掃引を行うとモードホップが起こり、連続的に広い波長範囲を掃引することはできない。 On the other hand, DFB-LD (Distributed FeedBack-Laser Diode) is generally known as a semiconductor laser capable of continuous wavelength sweeping. In order to change the output wavelength of the DFB-LD, sweeping of the injection current amount, temperature sweeping of the LD chip, etc. can be considered, but the variable wavelength range is very narrow, about 2 nm in the 1 μm band. Further, FP-LD (Fabry Perot-LD) is known as a laser having a wide wavelength sweep range. However, in FP-LD, when wavelength sweeping is performed, mode hops occur, and it is not possible to sweep a wide wavelength range continuously.
　このように、複数のガスをセンシングする光源には、広い波長範囲に渡って、密な掃引を行わなければならないという相反する特性が求められるとともに、小型で安価な光源が求められていた。 As described above, a light source for sensing a plurality of gases is required to have a contradictory characteristic that a dense sweep must be performed over a wide wavelength range, and a small and inexpensive light source.
　本発明の目的は、3μm帯における広い波長範囲に渡って、連続的に中赤外波長域の光を発生させることができる小型で安価な波長変換光源を提供することにある。 An object of the present invention is to provide a small and inexpensive wavelength conversion light source capable of continuously generating light in the mid-infrared wavelength region over a wide wavelength range in the 3 μm band.
　このような目的を達成するために、本発明の実施態様は、波長λ1の信号光を出力する第１の半導体レーザであって、前記信号光の波長を、所定の信号光波長範囲において連続的に可変することができる第１の半導体レーザと、波長λ3の励起光を出力する第２の半導体レーザであって、前記励起光の波長を、所定の励起光波長範囲において可変することができ、前記励起光波長範囲内において一定の間隔で波長飛びを有する第２の半導体レーザと、前記信号光と前記励起光とを合波する合波器と、前記合波器で合波された光を入射させ、前記信号光と前記励起光の差周波となる波長λ２の変換光を出力する非線形光学効果を有する非線型光学媒質とを備え、前記信号光波長範囲に相当する波数は、前記波長飛びの幅に相当する波数より広いことを特徴とする。 In order to achieve such an object, an embodiment of the present invention is a first semiconductor laser that outputs signal light having a wavelength λ 1 , wherein the wavelength of the signal light is continuously in a predetermined signal light wavelength range. A first semiconductor laser that can be variably changed and a second semiconductor laser that outputs excitation light having a wavelength λ 3 , wherein the wavelength of the excitation light can be varied within a predetermined excitation light wavelength range. The second semiconductor laser having a wavelength jump at a constant interval within the pumping light wavelength range, a multiplexer for multiplexing the signal light and the pumping light, and multiplexed by the multiplexer A non-linear optical medium having a nonlinear optical effect that makes light incident and outputs converted light having a wavelength λ 2 that is a difference frequency between the signal light and the excitation light, and the wave number corresponding to the signal light wavelength range is: It is wider than the wave number corresponding to the wavelength jump width. The features.
図１は、従来の波長変換を用いた光源の構成を示す概略図、FIG. 1 is a schematic diagram showing a configuration of a light source using conventional wavelength conversion, 図２は、位相不整合量に対する変換効率の変化を示す図、FIG. 2 is a diagram showing a change in conversion efficiency with respect to the phase mismatch amount; 図３は、LiNbO3の群屈折率の波長依存性を示す図、FIG. 3 is a diagram showing the wavelength dependence of the group refractive index of LiNbO 3 ; 図４は、本実施形態において、励起光を発生する半導体レーザの特性の一例を示す図、FIG. 4 is a diagram showing an example of characteristics of a semiconductor laser that generates excitation light in the present embodiment. 図５は、本実施形態において、信号光を発生する半導体レーザの特性の一例を示す図、FIG. 5 is a diagram illustrating an example of characteristics of a semiconductor laser that generates signal light in the present embodiment; 図６は、本発明の実施例１に係る構成の概念図を示す図、FIG. 6 is a diagram illustrating a conceptual diagram of the configuration according to the first embodiment of the invention. 図７は、本発明の実施例１において、信号光および励起光の波長を変化させた場合のLiNbO3導波路による波長変換の特性を示す図、FIG. 7 is a diagram showing the characteristics of wavelength conversion by the LiNbO 3 waveguide when the wavelengths of the signal light and the excitation light are changed in Example 1 of the present invention; 図８は、実施例１における変換光と信号光との関係を示す図、FIG. 8 is a diagram illustrating a relationship between converted light and signal light in the first embodiment. 図９は、本発明の実施例２に係る構成の概念図を示す図、FIG. 9 is a diagram illustrating a conceptual diagram of a configuration according to Embodiment 2 of the present invention. 図１０は、本発明の実施例３に係る構成の概念図を示す図、FIG. 10 is a diagram showing a conceptual diagram of a configuration according to Embodiment 3 of the present invention. 図１１は、実施例３において、励起光を発生する光源の特性の一例を示す図、FIG. 11 is a diagram illustrating an example of characteristics of a light source that generates excitation light in Example 3. 図１２Ａは、実施例４における信号光および励起光の波長範囲を示す図、FIG. 12A is a diagram showing the wavelength ranges of signal light and excitation light in Example 4, 図１２Ｂは、実施例４における信号光および変換光の波長範囲を示す図である。FIG. 12B is a diagram illustrating the wavelength ranges of signal light and converted light in the fourth embodiment.
　本願発明の発明者は、種々のガスが大きな吸収を示す3μm帯の波長域において、広い波長帯域に渡って出力の得られる光源の構成について鋭意検討を行った。ここでは、安価な光源を構成するために、励起光光源および信号光光源に、半導体レーザを用いることとした。信号光光源として、波長掃引が容易なDFB-LDを用いると、広い波長範囲での掃引が不可能であることから、励起光光源として、不連続に波長可変できる光源を組み合わせることとした。その結果、狭い波長帯域で密な波長掃引が可能な第１の半導体レーザとして、1.5μm帯のDFB-LDを用い、連続的ではなく、モードホップを許容して広い波長帯域で波長掃引が可能な第２の半導体レーザとして、1.0μm帯（1.0-1.1μmの範囲、より具体的には1.02-1.08μmの範囲）のFP-LDを用いる。第１の半導体レーザからの信号光と第２の半導体レーザからの励起光とを、周期分極反転を持つLiNbO3光導波路に入射して差周波発生を行うことにより、広い波長範囲で位相整合条件を保ち、3μm帯における広い波長範囲に渡って、中赤外波長域の光を発生させることができることを発見した。以下にその動作原理を説明する。 The inventor of the present invention diligently studied the configuration of a light source capable of obtaining an output over a wide wavelength band in a wavelength range of 3 μm where various gases exhibit large absorption. Here, in order to construct an inexpensive light source, semiconductor lasers are used for the excitation light source and the signal light source. When a DFB-LD with easy wavelength sweep is used as the signal light source, it is impossible to sweep in a wide wavelength range, so a light source capable of discontinuously varying the wavelength is used as the excitation light source. As a result, a 1.5μm band DFB-LD is used as the first semiconductor laser capable of dense wavelength sweeping in a narrow wavelength band, and wavelength sweeping is possible over a wide wavelength band allowing mode hops rather than continuous. As the second semiconductor laser, an FP-LD in the 1.0 μm band (1.0-1.1 μm range, more specifically 1.02-1.08 μm range) is used. Phase matching conditions over a wide wavelength range by generating the difference frequency by injecting the signal light from the first semiconductor laser and the excitation light from the second semiconductor laser into a LiNbO 3 optical waveguide having periodic polarization inversion It was discovered that light in the mid-infrared wavelength region can be generated over a wide wavelength range in the 3 μm band. The operation principle will be described below.
　擬似位相整合条件を保ったまま波長変換を行うためには、（式１）および（式２）に加え、次式を満たす必要がある。 In order to perform wavelength conversion while maintaining the quasi phase matching condition, in addition to (Expression 1) and (Expression 2), the following expression must be satisfied.
Figure JPOXMLDOC01-appb-M000004
Figure JPOXMLDOC01-appb-M000004
　信号光波長を固定し、励起光波長を変化させて変換光波長を変化させる場合、（式１）においてλ1が一定という条件から励起光と変換光の変化分は次式を満たす必要がある。 When the wavelength of the converted light is changed by fixing the wavelength of the signal light and changing the wavelength of the pump light, the change of the pump light and the converted light needs to satisfy the following formula from the condition that λ 1 is constant in (Equation 1). .
Figure JPOXMLDOC01-appb-M000005
Figure JPOXMLDOC01-appb-M000005
　（式２）および（式４）から、それぞれの波長における位相速度の変化分は次式を満たす必要がある。 From (Equation 2) and (Equation 4), the change in phase velocity at each wavelength must satisfy the following equation.
Figure JPOXMLDOC01-appb-M000006
Figure JPOXMLDOC01-appb-M000006
　（式５）および（式６）から、次式を満たす必要があることが分かる。 (Equation 5) and (Equation 6) show that the following equation must be satisfied.
Figure JPOXMLDOC01-appb-M000007
Figure JPOXMLDOC01-appb-M000007
　ここでngは、次式で与えられる群屈折率である。 Here, ng is a group refractive index given by the following equation.
Figure JPOXMLDOC01-appb-M000008
Figure JPOXMLDOC01-appb-M000008
　なお、群屈折率ngと群速度vgとの間には、ng=c/vgの関係が成り立つ。 Note that a relationship of n g = c / v g is established between the group refractive index ng and the group velocity v g .
　従って、（式７）から、励起光波長および変換光波長における群屈折率または群速度が等しければ、波長の変化に伴う伝播定数の変化が相殺され、（式２）の位相不整合量の変化が緩やかになる。この結果、広い波長域に渡って位相整合をとることが可能となる。 Accordingly, from (Equation 7), if the group refractive index or the group velocity at the pumping light wavelength and the converted light wavelength are equal, the change in the propagation constant accompanying the change in wavelength is canceled, and the change in the phase mismatch amount in (Equation 2) Becomes moderate. As a result, phase matching can be achieved over a wide wavelength range.
　本願発明の発明者は、非線形光学媒質として用いるLiNbO3の励起光波長および変換光波長における群屈折率について検討を行った。図３は、LiNbO3の群屈折率の波長依存性を示す図である。図３中の横軸は波長を表し、縦軸は群屈折率を表す。また、図３中の実線はバルク結晶のLiNbO3を用いた場合、点線は導波路構造を持つLiNbO3を用いた場合の結果を表す。 The inventor of the present invention examined the group refractive index of the excitation light wavelength and the conversion light wavelength of LiNbO 3 used as the nonlinear optical medium. FIG. 3 is a diagram showing the wavelength dependence of the group refractive index of LiNbO 3 . The horizontal axis in FIG. 3 represents the wavelength, and the vertical axis represents the group refractive index. Also, the solid line in FIG. 3 represents the result when using bulk crystal LiNbO 3 and the dotted line represents the result when using LiNbO 3 having a waveguide structure.
　図３より、メタンなどのガスの吸収がある3.3μm-3.6μmを中心とした波長域と群屈折率が一致するのは、1.0-1.1μmの間の波長域である。この波長域には、0.94μmから1.08μmの波長を出力するInGaAs系半導体レーザが存在する。Ybを添加した光ファイバを用いたYbファイバ増幅器を用いることを考えると、中心波長を1.05μmとし、1.02-1.08μm程度の可変波長範囲を有する半導体レーザを用いるのが適当である。この励起光の波長範囲に対し、中心波長が約3.5μmであって3.39～3.6μmの範囲で変換光と群屈折率が一致することが分かる。従って、信号光波長を1.50μm程度にすることで、群速度整合を利用することができる。これにより、FP-LDを用いた1.02-1.08μmの波長域の励起光を、3.5μmを中心とした広い帯域に渡る変換光に変換することができる。 From FIG. 3, it is the wavelength region between 1.0 and 1.1 μm that the group refractive index agrees with the wavelength region centered on 3.3 to 3.6 μm where gas such as methane is absorbed. In this wavelength region, there are InGaAs semiconductor lasers that output wavelengths from 0.94 μm to 1.08 μm. Considering the use of a Yb fiber amplifier using an optical fiber doped with Yb, it is appropriate to use a semiconductor laser having a center wavelength of 1.05 μm and a variable wavelength range of about 1.02-1.08 μm. With respect to the wavelength range of the excitation light, it can be seen that the converted light and the group refractive index coincide with each other when the center wavelength is about 3.5 μm and the range is 3.39 to 3.6 μm. Therefore, group velocity matching can be used by setting the signal light wavelength to about 1.50 μm. This makes it possible to convert the excitation light in the wavelength region of 1.02-1.08 μm using the FP-LD into converted light over a wide band centered on 3.5 μm.
　しかしながら、バルクのLiNbO3を用いる場合は光のパワー密度が大きくないために、大きな変換効率を得ることは難しい。一方、導波路構造を持つLiNbO3を用いた場合、光のパワー密度が大きいために、バルク結晶に比較して大きな変換効率を得ることができる。 However, when bulk LiNbO 3 is used, it is difficult to obtain a large conversion efficiency because the light power density is not large. On the other hand, when LiNbO 3 having a waveguide structure is used, since the power density of light is large, a large conversion efficiency can be obtained as compared with a bulk crystal.
　導波路を用いて1.0μm帯の励起光と1.55μm帯の信号光との差周波発生により3μmの変換光を得る場合、導波路の構造として、相互作用する３つの波長帯のうち最も長波長の3μmにおいてほぼシングルモード条件を満たすように、導波路のコアの大きさ等を設定する。この場合、1.0μm帯、1.55μm帯では導波路はマルチモードとなるため、波長変換に利用する基底モードは光導波路内に強く閉じ込められており、導波路の形状によって生じる構造分散の影響をほとんど受けない。従って、導波路構造を持つLiNbO3の群屈折率は1.0μm帯、1.55μm帯ではバルクの場合に比較してほとんど変化しない。それに対し、3μm帯では、導波モードの等価屈折率が、導波路構造による構造分散の影響を大きく受けることになる。よって、群速度の一致による特性は、3μm帯の群屈折率を計算することで解析することができる。 When using a waveguide to obtain 3μm converted light by generating a difference frequency between 1.0μm band excitation light and 1.55μm band signal light, the waveguide has the longest wavelength among the three interacting wavelength bands. The core size of the waveguide is set so that the single mode condition is almost satisfied at 3 μm. In this case, since the waveguide is multimode in the 1.0 μm band and 1.55 μm band, the fundamental mode used for wavelength conversion is strongly confined in the optical waveguide, and the influence of structural dispersion caused by the shape of the waveguide is almost eliminated. I do not receive it. Therefore, the group refractive index of LiNbO 3 having a waveguide structure hardly changes in the 1.0 μm band and the 1.55 μm band compared to the bulk case. On the other hand, in the 3 μm band, the equivalent refractive index of the waveguide mode is greatly affected by the structural dispersion due to the waveguide structure. Therefore, the characteristics due to the coincidence of group velocities can be analyzed by calculating the group refractive index in the 3 μm band.
　図３中の点線に示すように、導波路構造を付与することにより、屈折率の波長依存性（構造分散）が大きくなる。このため、（式８）の定義からも分かるように、導波路構造を持つ場合はバルク結晶に比較して、3μm帯における群屈折率が大きくなる。従って、図３から分かるように、Ybファイバ増幅器を用いた光源の典型的な波長可変範囲である1.02-1.08μmの波長帯に対して群速度の一致が得られる変換光の波長帯が短波長側へシフトすることになる。図３の例では、上記の励起波長に対して3.24-3.44μm(中心波長は3.34μm程度)の範囲で変換光と群屈折率が一致することが分かる。 As shown by the dotted line in FIG. 3, the wavelength dependence (structural dispersion) of the refractive index is increased by providing the waveguide structure. For this reason, as can be seen from the definition of (Equation 8), in the case of having a waveguide structure, the group refractive index in the 3 μm band is larger than that of the bulk crystal. Therefore, as can be seen from FIG. 3, the wavelength band of the converted light that provides the coincidence of group velocities with respect to the wavelength band of 1.02-1.08 μm, which is a typical wavelength variable range of a light source using a Yb fiber amplifier, is a short wavelength. Will shift to the side. In the example of FIG. 3, it can be seen that the converted light and the group refractive index coincide with each other in the range of 3.24 to 3.44 μm (the center wavelength is about 3.34 μm) with respect to the excitation wavelength.
　3μm帯では多くの炭化水素系ガスが強い光吸収を示し、例えば、典型的な炭化水素系ガスであるメタンは3.42μm、エタンは3.34μmにおいて吸収のピークが見られる。導波路構造を用いて波長変換を行うことによりこれらの吸収ピーク波長を１つの光源でカバーすることが可能になる。 In the 3 μm band, many hydrocarbon gases show strong light absorption. For example, methane, which is a typical hydrocarbon gas, has an absorption peak at 3.42 μm and ethane at 3.34 μm. By performing wavelength conversion using the waveguide structure, it is possible to cover these absorption peak wavelengths with one light source.
　ここで、具体的に群速度整合を用いた場合の差周波光（変換光）の掃引幅について述べる。上述したように、導波路長Ｌ=50mmとした場合、1.5μm帯の信号光の許容波長範囲は狭く、信号光の波長を掃引しても、3μm帯の差周波光の波長範囲では7nm程度である。これに対して、1.0μm帯の励起光波長を掃引すると、3μm帯の差周波光の波長範囲では123nm程度となる。群速度整合を用いる励起光の波長掃引の範囲は、信号光の波長掃引に対して、約17倍も波長域が広くなる。導波路長Ｌをさらに20mmと短くすると、式２および式３に従って、波長掃引幅はさらに広くなり、励起光波長を掃引すると、3μm帯の差周波光の波長範囲では197nm程度となる。このように、導波路長を短くすることにより許容波長範囲は広くなるが、同時に変換効率の低下を招く。変換効率の低下は、励起光光源にYbファイバ増幅器を組み合わせるなどして補償することができる。 Here, the sweep width of the difference frequency light (converted light) when the group velocity matching is specifically used will be described. As described above, when the waveguide length L is 50 mm, the allowable wavelength range of the signal light in the 1.5 μm band is narrow, and even if the wavelength of the signal light is swept, the wavelength range of the difference frequency light in the 3 μm band is about 7 nm. It is. On the other hand, when the excitation light wavelength in the 1.0 μm band is swept, the wavelength range of the difference frequency light in the 3 μm band is about 123 nm. The range of the wavelength sweep of the excitation light using the group velocity matching is approximately 17 times wider than the wavelength sweep of the signal light. When the waveguide length L is further shortened to 20 mm, the wavelength sweep width is further increased according to Equations 2 and 3, and when the excitation light wavelength is swept, the wavelength range of the difference frequency light in the 3 μm band is about 197 nm. As described above, the allowable wavelength range is widened by shortening the waveguide length, but at the same time, the conversion efficiency is lowered. The decrease in conversion efficiency can be compensated by combining a pumping light source with a Yb fiber amplifier.
　そこで、変換光の可変波長範囲を波数で100cm-1以上（波長範囲122nm以上）を目標とすると、信号光、励起光のいずれかを波数で100cm-1以上可変しなければならない。例えば、中心波長3.34μmの変換光の可変波長範囲を、波数に換算して100cm-1以上（波長範囲110nm以上）を目標とすると、1.55μm帯の信号光で24nm以上、または1.06μm帯の励起光で11nm以上の波長範囲で可変しなければならない。 Therefore, if the variable wavelength range of the converted light is targeted to be 100 cm -1 or more (wavelength range of 122 nm or more), either the signal light or the excitation light must be varied by 100 cm -1 or more. For example, if the variable wavelength range of converted light with a center wavelength of 3.34 μm is converted to wave number and the target is 100 cm −1 or more (wavelength range of 110 nm or more), the signal light of 1.55 μm band is 24 nm or more, or 1.06 μm band It must be tunable in the wavelength range of 11 nm or more with excitation light.
　図４に、励起光を発生する半導体レーザ（FP-LD）の特性の一例を示す。FP-LDへの電流注入量を一定にして、LDチップの温度を変化させた時の1.065μmを中心とするFP-LDの発振波長依存性を示す。5℃-40℃の温度変化により、1.05μm帯のDFB-LDは、通常2nm程度しか波長変化は起きないが、FP-LDでは11nm程度の波長変化が得られる。一方、図４からわかるように、0.5nm程度の波長飛びが、一定の間隔で20か所ほど観測されている。例えば、図４のプロットから、1060.0-1060.5nm＝0.5nmの波長飛び幅（モードホップ量）を、波数に換算すると、9434.0cm-1-9429.5cm-1＝4.5cm-1である。 FIG. 4 shows an example of characteristics of a semiconductor laser (FP-LD) that generates excitation light. This shows the dependence of the FP-LD on the oscillation wavelength centering on 1.065μm when the temperature of the LD chip is changed with the amount of current injected into the FP-LD constant. Due to the temperature change of 5 ° C-40 ° C, the 1.05μm band DFB-LD usually only changes the wavelength by about 2nm, but the FP-LD can change the wavelength by about 11nm. On the other hand, as can be seen from FIG. 4, about 20 nm wavelength jumps are observed at regular intervals. For example, from the plot of FIG. 4, 1060.0-1060.5nm = 0.5nm wavelength jump width (mode hop weight), in terms of wavenumber, a 9434.0cm -1 -9429.5cm -1 = 4.5cm -1.
　図５に、信号光を発生する半導体レーザ（DFB-LD）の特性の一例を示す。LDチップの温度を一定（25℃）とし、1.55μm帯DFB-LDへの電流注入量を変化させた場合の波長変化を示す。波長は連続的に掃引されており、図５のプロットから、その波長範囲は1566.9-1568.2nm＝1.3nm程度であり、波数に換算すると、6382.0cm-1-6376.6.cm-1＝5.4cm-1である。この波数量は励起光のモードホップ量に比して範囲が広く、励起光のモードホップを信号光の波長変化で補償することができる。位相整合範囲についても、導波路構造を持つLiNbO3の素子長を50mmと仮定すると、位相整合範囲の半値全幅は、1.55μm帯において1.4nmであり、十分許容できる。 FIG. 5 shows an example of characteristics of a semiconductor laser (DFB-LD) that generates signal light. This shows the change in wavelength when the temperature of the LD chip is constant (25 ° C) and the amount of current injected into the 1.55 µm band DFB-LD is changed. The wavelength is swept continuously. From the plot of FIG. 5, the wavelength range is about 1566.9-1568.2 nm = 1.3 nm, and when converted to wave number, 6382.0 cm −1 -6376.6.cm −1 = 5.4 cm − 1 . This wave quantity has a wider range than the mode hop amount of the pumping light, and the mode hop of the pumping light can be compensated by the wavelength change of the signal light. As for the phase matching range, assuming that the element length of LiNbO 3 having a waveguide structure is 50 mm, the full width at half maximum of the phase matching range is 1.4 nm in the 1.55 μm band, which is sufficiently acceptable.
　1.05μm帯励起光と3.4μm帯変換光との間で群速度整合を取った波長変換素子を用いると、広い励起光可変範囲が得られる。また、FP-LDなどのモードホップを有する励起光光源と連続波長掃引できる1.55μm帯信号光光源とを併用することにより、広い波長範囲で連続的に掃引が可能となる。 A wide excitation light variable range can be obtained by using a wavelength conversion element that matches the group velocity between the 1.05 μm band excitation light and the 3.4 μm band conversion light. Further, by using a pump light source having a mode hop such as FP-LD and a 1.55 μm band signal light source capable of continuous wavelength sweep, it is possible to continuously sweep over a wide wavelength range.
　図６に、本発明の一実施例に係る構成の概念図を示す。本実施例では、1.065μmを中心に波長可変なFP-LD１０２により励起光を発生させた。FP-LD１０２は、ペルチエ素子などの温度制御回路１０７により、出力光の波長を可変する。また、信号光の光源には、1.57μmを中心とするDFB-LD１０２を用い、駆動電流を制御することにより、出力光の波長掃引を行う。励起光と信号光をファイバカプラ１０３で合波し、周期分極反転構造の導波路１０５を有する非線形光学媒質であるLiNbO3結晶に入射して、差周波発生により3.3μmの中赤外光である変換光１０６を発生させた。 FIG. 6 shows a conceptual diagram of a configuration according to an embodiment of the present invention. In this example, excitation light was generated by the FP-LD 102 having a variable wavelength around 1.065 μm. The FP-LD 102 varies the wavelength of output light by a temperature control circuit 107 such as a Peltier element. Further, the DFB-LD 102 centered at 1.57 μm is used as the signal light source, and the wavelength of the output light is swept by controlling the drive current. The pumping light and the signal light are combined by the fiber coupler 103 and incident on a LiNbO 3 crystal, which is a nonlinear optical medium having a waveguide 105 with a periodically poled structure, and is 3.3 μm mid-infrared light due to difference frequency generation. Converted light 106 was generated.
　次に、波長変換素子１０４の詳細について説明する。波長変換素子１０４は、導波路１０５を有する非線形光学媒質、および光の入出力のためのレンズ等を有する。非線形光学媒質は、非特許文献３に示されるウエハ接合法によって作製されている。コアにはZnを7mol%添加したLiNbO3、クラッドにはLiTaO3を用い、ダイシングによりリッジ型の光導波路を形成してある。LiNbO3の分極反転構造および導波路構造の分散により位相整合特性すなわち波長変換可能な波長帯域特性が決定される。 Next, details of the wavelength conversion element 104 will be described. The wavelength conversion element 104 includes a nonlinear optical medium having a waveguide 105, a lens for inputting and outputting light, and the like. The nonlinear optical medium is manufactured by a wafer bonding method shown in Non-Patent Document 3. A ridge-type optical waveguide is formed by dicing using LiNbO 3 doped with 7 mol% of Zn for the core and LiTaO 3 for the clad. The phase matching characteristic, that is, the wavelength band characteristic capable of wavelength conversion is determined by the dispersion of the polarization inversion structure and waveguide structure of LiNbO 3 .
　以下に導波路構造、分極反転構造と位相整合特性の詳細を説明する。本実施例ではコアを形成するLiNbO3のサイズを厚み10μm、幅14μmに設定した。この導波路サイズにおける導波路の群屈折率の分散は図３に示したとおりである。本実施例では中心波長が1.065μmである励起光と中心波長が3.31μmである変換光との間で群速度整合が満たされ、励起光、信号光、変換光の３者の間で位相整合が同時に満たされるように素子を設計している。本実施例では分極反転の基本周期をΛ0=28.16μmとし、素子長をL=50mmとした。 Details of the waveguide structure, polarization inversion structure, and phase matching characteristics will be described below. In this example, the size of LiNbO 3 forming the core was set to a thickness of 10 μm and a width of 14 μm. The dispersion of the group refractive index of the waveguide in this waveguide size is as shown in FIG. In this embodiment, group velocity matching is satisfied between pump light having a center wavelength of 1.065 μm and converted light having a center wavelength of 3.31 μm, and phase matching is performed between the pump light, signal light, and converted light. The device is designed so that is simultaneously satisfied. In this example, the basic period of polarization inversion was Λ 0 = 28.16 μm, and the element length was L = 50 mm.
　図７に、本実施例で信号光および励起光の波長を変化させた場合のLiNbO3導波路による波長変換の特性を示す。図７において、縦軸は規格化された変換効率を示し、横軸は変換光波長を示す。信号光の波長を1.570μmに固定し、FP-LDを、1.049μmから1.078μmの範囲で波長可変した場合の変換特性（図７中の実線）である。本実施例では励起光と変換光との間での群速度整合を利用できるため、図７に示すように、励起光の波長可変範囲の全域に渡って差周波発生を効率良く行うことが可能であった。また、本実施例では、3.24μmから3.36μmまでの120nmの範囲に渡って変換光を出力することが可能であった。 FIG. 7 shows the characteristics of wavelength conversion by the LiNbO 3 waveguide when the wavelengths of the signal light and the excitation light are changed in this example. In FIG. 7, the vertical axis indicates the normalized conversion efficiency, and the horizontal axis indicates the converted light wavelength. This is a conversion characteristic (solid line in FIG. 7) when the wavelength of the signal light is fixed to 1.570 μm and the wavelength of the FP-LD is varied in the range of 1.049 μm to 1.078 μm. In this embodiment, since group velocity matching between the pumping light and the converted light can be used, as shown in FIG. 7, it is possible to efficiently generate the difference frequency over the entire wavelength variable range of the pumping light. Met. In this example, it was possible to output the converted light over a range of 120 nm from 3.24 μm to 3.36 μm.
　一方、励起光の波長を1.059μmから1.071μmまでの間の様々な波長に固定して、信号光波長を変化させた場合の波長変換特性も合わせて示す（図７中の(a)～(s)）。DFB-LDの信号光波長を変化させた場合、波長変換が可能な帯域は信号光の波長にして1.4nm、変換光の波長にして6nm程度であり、図７に示したFP-LDの波長可変帯域の1/20程度であった。この波長変換特性の曲線は、信号光の変化により（式２）の位相不整合量がほぼ直線的に変化するために、図２に示した位相整合曲線と同様の形状になる。 On the other hand, the wavelength conversion characteristics when the wavelength of the excitation light is fixed to various wavelengths between 1.059 μm and 1.071 μm and the signal light wavelength is changed are also shown ((a) to (in FIG. 7). s)). When the DFB-LD signal light wavelength is changed, the wavelength-convertible band is 1.4 nm for the signal light wavelength and about 6 nm for the converted light wavelength. The wavelength of the FP-LD shown in FIG. It was about 1/20 of the variable bandwidth. This wavelength conversion characteristic curve has the same shape as the phase matching curve shown in FIG. 2 because the amount of phase mismatch in (Equation 2) changes almost linearly due to changes in signal light.
　図８に、実施例１における変換光と信号光との関係を示す。1.05μm帯のFP-LDのモードホップの谷間を埋めるように、1.55μm帯のDFB-LDを変調する。本実施例では、DFB-LDからの信号光の波長を、1.5694μmから1.5708μmの狭い範囲で掃引するだけで、励起光のモードホップを補償できる十分な波長変化を有するので、3.24-3.36μmの広い波長範囲にわたって、連続的に波長掃引できる。 FIG. 8 shows the relationship between the converted light and the signal light in the first embodiment. Modulates the DFB-LD in the 1.55 μm band to fill the valley of the mode hop in the FP-LD in the 1.05 μm band. In this embodiment, the wavelength of the signal light from the DFB-LD has a sufficient wavelength change that can compensate for the mode hop of the pumping light only by sweeping in a narrow range from 1.5694 μm to 1.5708 μm. The wavelength can be swept continuously over a wide wavelength range.
　これらの結果より、本実施例の有効性が確認できた。本実施例では励起光の波長を掃引することにより、例えばガスの吸収スペクトルを120nmの範囲に渡って測定することが可能である。 From these results, the effectiveness of this example could be confirmed. In the present embodiment, by sweeping the wavelength of the excitation light, for example, the absorption spectrum of the gas can be measured over a range of 120 nm.
　本実施例に用いた波長変換素子の変換効率は20%/Wである。信号光として20mW、励起光として400mWを素子に入力した結果、0.8mWの出力が得られ、ガスの検出応用に十分な出力が得られた。 The conversion efficiency of the wavelength conversion element used in this example is 20% / W. As a result of inputting 20 mW as signal light and 400 mW as excitation light into the device, an output of 0.8 mW was obtained, which was sufficient for gas detection applications.
　本実施例では、導波路のコアとしてZnを添加したLiNbO3を用いた。Znを添加したLiNbO3を用いることにより、特に短波長の励起光の強度が大きい場合の光損傷を防ぐことができる。光損傷を防ぐ目的でZn以外にもMg、Sc、Inなどを添加したLiNbO3を同様に用いることもできる。本実施例で用いたZnはLiNbO3の屈折率を上昇させることが知られているが、MgなどはLiNbO3の屈折率を低下させることが知られている。従って、Zn以外の添加物を用いた場合は、材料自体の波長分散さらには導波路の閉じ込めが変化することにより構造分散が変化するために、群速度整合が得られる波長が変化する。この性質を積極的に利用して所望の波長において群速度整合が得られるように導波路の材料構成やコアのサイズを変更しても良い。 In this example, LiNbO 3 doped with Zn was used as the core of the waveguide. By using LiNbO 3 to which Zn is added, it is possible to prevent optical damage especially when the intensity of the short-wavelength excitation light is large. LiNbO 3 to which Mg, Sc, In or the like is added in addition to Zn for the purpose of preventing photodamage can also be used. Zn used in this example is known to increase the refractive index of LiNbO 3 , but Mg and the like are known to decrease the refractive index of LiNbO 3 . Therefore, when an additive other than Zn is used, the wavelength at which group velocity matching is obtained changes because the structural dispersion changes due to the change in the wavelength dispersion of the material itself and the confinement of the waveguide. The material configuration of the waveguide and the size of the core may be changed so as to obtain group velocity matching at a desired wavelength by actively utilizing this property.
　ガスの検出方法によっては、さらに高出力の3μm帯の出力光が必要となる場合がある。実施例１では、励起光および信号光の光源に半導体レーザのみを用いたが、励起光をYbファイバ増幅器を用いて増幅したり、信号光をEr添加ファイバ増幅器を用いて増幅することもできる。励起光および信号光の両方を、ファイバ増幅器を用いて増幅してもよい。 Depending on the gas detection method, higher output light in the 3 μm band may be required. In the first embodiment, only the semiconductor laser is used as the light source for the pumping light and the signal light. However, the pumping light can be amplified using a Yb fiber amplifier, or the signal light can be amplified using an Er-doped fiber amplifier. Both excitation light and signal light may be amplified using a fiber amplifier.
　図９に、本発明の別の実施例に係る構成の概念図を示す。本実施例の構成は、実施例１の構成とほぼ同様であるが、1.07μmを中心に波長可変なFP-LD２０１の出力をYbファイバ増幅器２０８で増幅して励起光を発生させた点において相違する。また、DFB-LD２０２の波長1.589μmの出力を、Lバンド用Er添加ファイバ増幅器２０９で増幅して信号光を発生させた。励起光と信号光をファイバカプラ２０３で合波し、周期分極反転構造の導波路２０５を有する非線形光学媒質であるLiNbO3結晶に入射して、差周波発生により3.275μmを中心とする中赤外光である変換光２０６を発生させた。 FIG. 9 shows a conceptual diagram of a configuration according to another embodiment of the present invention. The configuration of the present embodiment is almost the same as the configuration of the first embodiment, but differs in that the pumping light is generated by amplifying the output of the tunable FP-LD 201 centering on 1.07 μm by the Yb fiber amplifier 208. To do. The output of the DFB-LD 202 having a wavelength of 1.589 μm was amplified by an L-band Er-doped fiber amplifier 209 to generate signal light. The excitation light and the signal light are combined by the fiber coupler 203 and incident on a LiNbO 3 crystal which is a nonlinear optical medium having a waveguide 205 with a periodically poled structure, and a mid-infrared centered at 3.275 μm by difference frequency generation. Converted light 206, which is light, was generated.
　次に、波長変換素子２０４の詳細について説明する。本実施例に係る波長変換素子２０４は実施例１とほぼ同様であるが、本実施例では分極反転の基本周期をΛ0=28.3μmに設定した。また、本実施例では、励起光波長1.07μmと変換光波長3.275μmとの間で群速度整合が得られるように素子を設計した。 Next, details of the wavelength conversion element 204 will be described. The wavelength conversion element 204 according to this example is substantially the same as that of Example 1, but in this example, the fundamental period of polarization inversion was set to Λ 0 = 28.3 μm. In this example, the element was designed so that group velocity matching was obtained between the excitation light wavelength of 1.07 μm and the converted light wavelength of 3.275 μm.
　本実施例で用いた波長可変のFP-LDは、1.064μmから1.076μmの範囲で波長可変である。本実施例では励起光と変換光との間での群速度整合が利用できるため、励起光の波長可変範囲の全域に渡って差周波発生を効率良く行うことが可能であった。また、本実施例では、3.22μmから3.33μmまでの110nmの範囲に渡って変換光を出力することが可能であった。このように、本発明によれば、FP-LDの発振可能な波長の中から任意の波長帯を選んで、変換光との群速度整合を利用して広帯域の中赤外光を発生することができる。さらに、信号光波長もEr添加ファイバ増幅器が利用可能な波長を用いて群速度整合が取れるために効率良く波長変換を行うことができる。 The tunable FP-LD used in this example is tunable in the range of 1.064 μm to 1.076 μm. In this embodiment, since group velocity matching between the pumping light and the converted light can be used, it is possible to efficiently generate the difference frequency over the entire wavelength variable range of the pumping light. In this example, it was possible to output converted light over a 110 nm range from 3.22 μm to 3.33 μm. Thus, according to the present invention, an arbitrary wavelength band is selected from the oscillatable wavelengths of the FP-LD, and broadband mid-infrared light is generated using group velocity matching with the converted light. Can do. Furthermore, the signal light wavelength can be converted efficiently because the group velocity matching is achieved using a wavelength that can be used by the Er-doped fiber amplifier.
　図１０に、本発明の別の実施例に係る構成の概念図を示す。実施例１および実施例２では、FP-LDを励起光光源として用いたが、本実施例では、励起光光源３０１としてMEMSミラーを用いた外部共振器型の半導体レーザを用いた点で相違する。励起光光源３０１は、通常の半導体レーザの共振器を構成する一方の端面を、無反射コーティングを付した半導体増幅媒質３１１を備え、無反射コーティングを付した端面側に、レンズ３１２、MEMSミラー３１３およびグレーティング３１４が配置された外部共振器を備える。このような外部共振器型の半導体レーザは、位相調整機構を持たない簡易な構成であり、かつ波長選択のための駆動部にMEMSミラーを用いることにより、小型で安価な光源を実現することができる。実施例３の光源の構成は、いわゆるリトロー配置といわれる構成であるが、リットマン配置としても原理的には同じ機能を奏することができる。 FIG. 10 shows a conceptual diagram of a configuration according to another embodiment of the present invention. In the first and second embodiments, the FP-LD is used as a pumping light source. However, the present embodiment is different in that an external resonator type semiconductor laser using a MEMS mirror is used as the pumping light source 301. . The excitation light source 301 includes a semiconductor amplifying medium 311 with one end face constituting a resonator of a normal semiconductor laser provided with a nonreflective coating, and a lens 312 and a MEMS mirror 313 on the end face side provided with a nonreflective coating. And an external resonator in which a grating 314 is disposed. Such an external resonator type semiconductor laser has a simple configuration without a phase adjustment mechanism, and can realize a small and inexpensive light source by using a MEMS mirror as a drive unit for wavelength selection. it can. The configuration of the light source of the third embodiment is a so-called Littrow arrangement, but the same function can be achieved in principle even with the Littman arrangement.
　図１１に、実施例３において、励起光を発生する光源の特性の一例を示す。半導体レーザの中心波長は1.064μmであり、MEMSミラーを制御して、グレーティングに入射する光の角度を変化させる。これにより、グレーティングの反射波長特性が変化して、出力される励起光波長を1.059μmから1.069μmの範囲で可変することができる。外部共振器を構成しているので、通常の半導体レーザよりも共振器長が長くなり、波長飛びを生ずる間隔は、FP-LDよりも狭くなっている。 FIG. 11 shows an example of characteristics of a light source that generates excitation light in the third embodiment. The center wavelength of the semiconductor laser is 1.064 μm, and the MEMS mirror is controlled to change the angle of light incident on the grating. As a result, the reflected wavelength characteristic of the grating changes, and the output excitation light wavelength can be varied in the range of 1.059 μm to 1.069 μm. Since the external resonator is configured, the resonator length is longer than that of a normal semiconductor laser, and the interval at which the wavelength jump occurs is narrower than that of the FP-LD.
　また、DFB-LD３０２の波長1.570μmの出力を、Lバンド用Er添加ファイバ増幅器３０９で増幅して信号光を発生させた。励起光と信号光をファイバカプラ３０３で合波し、周期分極反転構造の導波路３０５を有する非線形光学媒質であるLiNbO3結晶を有する波長変換素子３０４に入射して、3.30μmを中心とする中赤外光である変換光３０６を発生させた。本実施例においては、周期分極反転構造の導波路３０５は、実施例１と同じ構造であり、分極反転の基本周期をΛ0=28.16μmに設定した。 Further, the output of the DFB-LD 302 having a wavelength of 1.570 μm was amplified by an L-band Er-doped fiber amplifier 309 to generate signal light. The excitation light and the signal light are combined by the fiber coupler 303 and incident on a wavelength conversion element 304 having a LiNbO 3 crystal, which is a nonlinear optical medium having a waveguide 305 having a periodically poled structure, and centering around 3.30 μm. Converted light 306 that is infrared light was generated. In the present embodiment, the waveguide 305 having a periodically poled structure has the same structure as that of the first embodiment, and the basic period of polarization inversion is set to Λ 0 = 28.16 μm.
　実施例３においても、MEMSミラーの角度を調節するMEMSパラメータを制御して、モードホップを含むが、広い波長範囲で励起光の波長を変化させ、DFB-LDの駆動電流を連続的に変化させることにより、3μm帯の変換光を得ることができる。MEMSミラーを用いた外部共振器型レーザのモードホップ量は、FP-LDのモードホップ量よりも狭いので、DFB-LDの駆動電流の変化量は、実施例１の場合と比較して小さくすることができる。 Also in the third embodiment, the MEMS parameter for adjusting the angle of the MEMS mirror is controlled to include the mode hop, but the wavelength of the excitation light is changed in a wide wavelength range, and the driving current of the DFB-LD is continuously changed. Thus, converted light in the 3 μm band can be obtained. Since the mode hop amount of the external cavity laser using the MEMS mirror is narrower than the mode hop amount of the FP-LD, the amount of change in the drive current of the DFB-LD is made smaller than that in the first embodiment. be able to.
　図１２Ａに、実施例４における信号光および励起光の波長範囲を示す。基本周期Λの分極反転構造を有し、厚み10μm、幅14μmの導波路構造を有するLiNbO3導波路であり、導波路長L=50mmの場合について、基本周期を27.177μmから28.607μmとし、信号光波長を1.47μmから1.59μmに変化させ、励起光波長を変化させたときの変換特性である。 FIG. 12A shows the wavelength ranges of the signal light and the excitation light in Example 4. LiNbO 3 waveguide having a polarization inversion structure with a basic period Λ, a waveguide structure with a thickness of 10 μm and a width of 14 μm, and for a waveguide length L = 50 mm, the basic period is changed from 27.177 μm to 28.607 μm, This is a conversion characteristic when the light wavelength is changed from 1.47 μm to 1.59 μm and the excitation light wavelength is changed.
　図１２Ｂに、実施例４における信号光および変換光の波長範囲を示す。例えば、分極反転構造の基本周期Λ=27.177μmの場合、励起光波長1.028μmと変換光波長3.42μmにおいて群速度整合を実現することができる。このとき、信号光波長は、式１により決定され、1.47μmとなる。信号光波長を1.47μmに固定し、励起光波長を1.028μmを中心に変化させると、変換光は、3.36μmから3.49μmの130nmの範囲にわたって変化する。 FIG. 12B shows the wavelength ranges of signal light and converted light in Example 4. For example, when the fundamental period Λ = 27.177 μm of the domain-inverted structure, group velocity matching can be realized at the excitation light wavelength of 1.028 μm and the converted light wavelength of 3.42 μm. At this time, the signal light wavelength is determined by Equation 1 and is 1.47 μm. When the signal light wavelength is fixed at 1.47 μm and the excitation light wavelength is changed around 1.028 μm, the converted light changes over a range of 130 nm from 3.36 μm to 3.49 μm.
　同様にして、分極反転構造の基本周期Λを適宜変更し、群速度整合がとれる励起光と変換光の組合せによって、信号光の波長を1.47μmから1.59μmの間で選択すると、励起光波長を1.02μmから1.08μmの間で変化させても、広い波長変換帯域を得ることができる。 Similarly, if the fundamental period Λ of the domain-inverted structure is appropriately changed and the wavelength of the signal light is selected from 1.47 μm to 1.59 μm by the combination of the excitation light and the converted light that can achieve group velocity matching, the excitation light wavelength is changed. Even if it is changed between 1.02 μm and 1.08 μm, a wide wavelength conversion band can be obtained.
　すなわち、Ybファイバ増幅器を用いることができる励起光波長1.02μm≦λ3≦1.08μmの範囲において、信号光波長を1.47μm≦λ1≦1.59μmの範囲で適宜選択すれば、変換光（差周波光）波長3.2μm≦λ2≦3.5μmを得ることができる。このとき、実施例１～３と同様に、励起光波長を広く掃引することができる光源として、モードホップを生ずる半導体レーザを用いることができる。連続掃引は困難であるが、信号光の波長を狭い範囲で連続的に掃引することにより、励起光のモードホップを補償することができ、全体として、連続掃引のできる広い波長帯域を有する3μm帯の波長変換光源を実現することができる。 That is, if the signal light wavelength is appropriately selected within the range of 1.47 μm ≦ λ 1 ≦ 1.59 μm in the pumping light wavelength range of 1.02 μm ≦ λ 3 ≦ 1.08 μm that can use the Yb fiber amplifier, the converted light (difference frequency) Light) wavelength 3.2 μm ≦ λ 2 ≦ 3.5 μm can be obtained. At this time, as in the first to third embodiments, a semiconductor laser that generates a mode hop can be used as a light source that can sweep the excitation light wavelength widely. Although continuous sweeping is difficult, the mode hop of the excitation light can be compensated by continuously sweeping the wavelength of the signal light in a narrow range, and as a whole, the 3 μm band has a wide wavelength band that allows continuous sweeping The wavelength conversion light source can be realized.
　また、信号光は、1.4μm帯においては、Tm添加ファイバ増幅器を用いることができ、1.5μm帯においては、Er添加ファイバ増幅器を用いることができ、変換光の出力を増強することができる。また、1.53μmから1.59μmの波長範囲は光通信に利用されており、安価なEr添加ファイバ増幅器を用いることができる。このとき、励起光波長は、1.05μmから1.08μmの波長範囲となる。本実施形態においては、波長可変できるレーザとして半導体レーザを用いるが、この波長帯は加工用ファイバレーザのシード光の光源として用いられるレーザの入手が可能であるので、容易に光源を構成することができる。 Also, the signal light can use a Tm-doped fiber amplifier in the 1.4 μm band, and can use an Er-doped fiber amplifier in the 1.5 μm band, thereby enhancing the output of the converted light. The wavelength range of 1.53 μm to 1.59 μm is used for optical communication, and an inexpensive Er-doped fiber amplifier can be used. At this time, the excitation light wavelength is in the wavelength range of 1.05 μm to 1.08 μm. In this embodiment, a semiconductor laser is used as a laser whose wavelength can be varied. However, since this wavelength band is available as a seed light source for a processing fiber laser, a light source can be easily configured. it can.
　本発明によれば、差周波発生によって変換光を得る波長変換素子において、LiNbO3導波路の分散特性を利用して1.0μm帯の励起光と3μm帯の変換光との間で群速度整合を得ることが出来る。これにより、3μm帯において幅広い帯域を有する変換光を発生可能な波長変換素子を実現することができる。 According to the present invention, in a wavelength conversion element that obtains converted light by generating difference frequency, group velocity matching is performed between 1.0 μm band excitation light and 3 μm band converted light using the dispersion characteristics of the LiNbO 3 waveguide. Can be obtained. Thereby, a wavelength conversion element capable of generating converted light having a wide band in the 3 μm band can be realized.
　また、信号光の中心波長を所定の信号光波長範囲において連続的に可変することができる第１の半導体レーザと、励起光の波長を所定の励起光波長範囲で可変することができ、この励起光波長範囲内において一定の間隔で波長飛びを有する第２の半導体レーザとを備え、波長飛びの幅より広い信号光波長範囲を設定したので、励起光の波長と信号光の波長とを変化させることにより、3μm帯の広い波長範囲にわたって連続的に密な掃引を行うことができる。 In addition, the first semiconductor laser capable of continuously varying the center wavelength of the signal light in a predetermined signal light wavelength range, and the wavelength of the pump light can be varied in the predetermined pump light wavelength range. A second semiconductor laser having wavelength jumps at regular intervals within the optical wavelength range, and a signal light wavelength range wider than the width of the wavelength jump is set, so that the wavelength of the excitation light and the wavelength of the signal light are changed. Thus, it is possible to perform dense sweep continuously over a wide wavelength range of 3 μm band.

Claims (11)

  1. 　波長λ1の信号光を出力する第１の半導体レーザであって、前記信号光の波長を、所定の信号光波長範囲において連続的に可変することができる第１の半導体レーザと、
    　波長λ3の励起光を出力する第２の半導体レーザであって、前記励起光の波長を、所定の励起光波長範囲において可変することができ、前記励起光波長範囲内において一定の間隔で波長飛びを有する第２の半導体レーザと、
    　前記信号光と前記励起光とを合波する合波器と、
    　前記合波器で合波された光を入射させ、前記信号光と前記励起光の差周波となる波長λ２の変換光を出力する非線形光学効果を有する非線型光学媒質とを備え、
    　前記信号光波長範囲に相当する波数は、前記波長飛びの幅に相当する波数より広いことを特徴とする波長変換光源。     A first semiconductor laser that outputs signal light having a wavelength λ 1 , wherein the wavelength of the signal light can be continuously varied in a predetermined signal light wavelength range;
    A second semiconductor laser for outputting pumping light of wavelength lambda 3, the wavelength of the excitation light, can be varied in a given excitation light wavelength range, the wavelength at regular intervals in the excitation light wavelength range A second semiconductor laser having a jump;
    A multiplexer for multiplexing the signal light and the excitation light;
    A non-linear optical medium having a non-linear optical effect that makes the light combined by the multiplexer incident and outputs converted light having a wavelength λ 2 that is a difference frequency between the signal light and the excitation light,
    The wavelength conversion light source, wherein a wave number corresponding to the signal light wavelength range is wider than a wave number corresponding to the width of the wavelength jump.
  2. 　前記非線型光学媒質は、光導波路構造を有し、前記光導波路における前記励起光の群速度と前記光導波路における前記変換光の群速度とが等しいことを特徴とする請求項１に記載の波長変換光源。 2. The wavelength according to claim 1, wherein the nonlinear optical medium has an optical waveguide structure, and the group velocity of the excitation light in the optical waveguide is equal to the group velocity of the converted light in the optical waveguide. Conversion light source.
  3. 　前記非線形光学媒質は、非線形光学材料の分極を周期的に反転した構造を有することを特徴とする請求項１または２に記載の波長変換光源。 The wavelength conversion light source according to claim 1 or 2, wherein the nonlinear optical medium has a structure in which polarization of a nonlinear optical material is periodically inverted.
  4. 　前記非線形光学材料は、LiNbO3または前記LiNbO3にMg、Zn、Sc、およびInからなる群から選ばれた少なくとも一種が添加物として含有された材料から成ることを特徴とする請求項３に記載の波長変換光源。 The nonlinear optical material is made of LiNbO 3 or a material containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive in LiNbO 3. Wavelength conversion light source.
  5. 　前記信号光波長範囲は、1.47μm≦λ1≦1.59μmの範囲であり、
    　前記励起光波長範囲は、1.02μm≦λ3≦1.08μmの範囲であることを特徴とする請求項１ないし４のいずれかに記載の波長変換光源。     The signal light wavelength range is a range of 1.47 μm ≦ λ 1 ≦ 1.59 μm,
    5. The wavelength-converted light source according to claim 1, wherein the wavelength range of the excitation light is 1.02 μm ≦ λ 3 ≦ 1.08 μm.
  6. 　前記第１の半導体レーザは、DFB-LDであることを特徴とする請求項１ないし５のいずれかに記載の波長変換光源。 The wavelength conversion light source according to any one of claims 1 to 5, wherein the first semiconductor laser is a DFB-LD.
  7. 　前記信号光は、光ファイバ増幅器を用いて増幅された光であることを特徴とする請求項１ないし６のいずれかに記載の波長変換光源。 The wavelength conversion light source according to any one of claims 1 to 6, wherein the signal light is light amplified by using an optical fiber amplifier.
  8. 　前記第２の半導体レーザは、FP-LDであることを特徴とする請求項１ないし７のいずれかに記載の波長変換光源。 The wavelength conversion light source according to any one of claims 1 to 7, wherein the second semiconductor laser is an FP-LD.
  9. 　前記第２の半導体レーザは、半導体増幅媒質と、外部共振器を構成するレンズ、MEMSミラーおよびグレーティングとから構成されていることを特徴とする請求項１ないし７のいずれかに記載の波長変換光源。 8. The wavelength-converted light source according to claim 1, wherein the second semiconductor laser is composed of a semiconductor amplification medium, a lens constituting an external resonator, a MEMS mirror, and a grating. .
  10. 　前記励起光は、Ybを添加した光ファイバ増幅器を用いて増幅された光であることを特徴とする請求項１ないし９のいずれかに記載の波長変換光源。 10. The wavelength conversion light source according to claim 1, wherein the excitation light is light amplified using an optical fiber amplifier to which Yb is added.
  11. 　波長λ1の信号光と波長λ3の励起光とを合波し、合波された光を、非線形光学効果を有する非線型光学媒質に入射し、前記信号光と前記励起光の差周波となる波長λ２の変換光を発生させる波長変換光源における方法であって、
    　前記信号光を出力する第１の半導体レーザを制御して、前記信号光の波長を、所定の信号光波長範囲において連続的に可変すること、および
    　前記励起光を出力する第２の半導体レーザを制御して、前記励起光の波長を、所定の励起光波長範囲において可変することであって、該第２の半導体レーザは、前記励起光波長範囲内において一定の間隔で波長飛びを有し、前記信号光波長範囲に相当する波数は、前記波長飛びの幅に相当する波数より広いこと
    　を備えたことを特徴とする方法。     The signal light of wavelength λ 1 and the excitation light of wavelength λ 3 are combined, the combined light is incident on a nonlinear optical medium having a nonlinear optical effect, and the difference frequency between the signal light and the excitation light is A wavelength conversion light source for generating converted light of wavelength λ 2
    Controlling the first semiconductor laser that outputs the signal light to continuously vary the wavelength of the signal light in a predetermined signal light wavelength range; and a second semiconductor laser that outputs the pumping light. Controlling the wavelength of the pumping light to be varied within a predetermined pumping light wavelength range, wherein the second semiconductor laser has wavelength jumps at regular intervals within the pumping light wavelength range; The wave number corresponding to the signal light wavelength range is wider than the wave number corresponding to the width of the wavelength jump.
PCT/JP2014/003019 2013-06-07 2014-06-06 Wavelength conversion light source WO2014196207A1 (en)

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JP2006227300A (en) * 2005-02-17 2006-08-31 Oki Electric Ind Co Ltd Wavelength conversion device
JP2007079227A (en) * 2005-09-15 2007-03-29 Nippon Telegr & Teleph Corp <Ntt> Wavelength conversion element and variable wavelength light source
JP2010066380A (en) * 2008-09-09 2010-03-25 Toyota Central R&D Labs Inc Wavelength variable terahertz wave generating apparatus
JP2011091209A (en) * 2009-10-22 2011-05-06 Sun Tec Kk Wavelength scanning type laser light source
JP2011203376A (en) * 2010-03-24 2011-10-13 Nippon Telegr & Teleph Corp <Ntt> Wavelength conversion element and wavelength conversion light source

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS62244184A (en) * 1986-04-16 1987-10-24 Tokyo Optical Co Ltd Oscillating frequency and output stabilizer for semiconductor laser
JP2006227300A (en) * 2005-02-17 2006-08-31 Oki Electric Ind Co Ltd Wavelength conversion device
JP2007079227A (en) * 2005-09-15 2007-03-29 Nippon Telegr & Teleph Corp <Ntt> Wavelength conversion element and variable wavelength light source
JP2010066380A (en) * 2008-09-09 2010-03-25 Toyota Central R&D Labs Inc Wavelength variable terahertz wave generating apparatus
JP2011091209A (en) * 2009-10-22 2011-05-06 Sun Tec Kk Wavelength scanning type laser light source
JP2011203376A (en) * 2010-03-24 2011-10-13 Nippon Telegr & Teleph Corp <Ntt> Wavelength conversion element and wavelength conversion light source

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